1. Technical Field
The present invention relates to an electrostatic capacitance detection device that reads the surface contours of a fingerprint or other target object having extremely small ridges and valleys by detecting electrostatic capacitance, which changes according to the distance from the surface of the target object.
2. Related Art
Conventionally, electrostatic capacitance detection devices used for fingerprint sensors, etc., are fabricated so that a sensor electrode and a protective film such as a dielectric provided on the sensor electrode are formed on a substrate as described in examples of related art listed as follows: Japanese Unexamined Patent Publication No. 2000-346610 is a first example of related art; Japanese Unexamined Patent Publication No. 2003-254706 is a second example of related art; Japanese Unexamined Patent Publication No. 11-118415 is a third example of related art; Japanese Unexamined Patent Publication No. 2000-346608 is a fourth example of related art; Japanese Unexamined Patent Publication No. 2001-56204 is a fifth example of related art; and Japanese Unexamined Patent Publication No. 2001-133213 is a sixth example of related art.
Here, in the electrostatic capacitance detection device, the thickness of dielectric layer cannot be increased from electrostatic capacitance detection point of view. In addition, a target object to be detected is usually charged. When the target object having a charge, it often causes a breakdown of a peripheral circuit due to an instantaneous current induced by a discharge.
In order to avoid the breakdown due to the instantaneous current, for example, the first example of related art describes the following manner: a sensing electrode is formed in a layer different from the layer in which a conversion circuit and wirings are formed; the sensing electrode is disposed at a location adjacent to the target object; and the conversion circuit and wirings are disposed as a lower layer of insulator, the sensing electrode, and the protective film. Also, in order to prevent a sensor from a breakdown, the second example of related art describes a configuration in which a conductor or cover, which the target object comes in contact with before contacting the sensor, is provided so as to flow a charge to a power supply (refer to its paragraphs 0077 to 0081).
As above-mentioned, since the thickness of the dielectric layer cannot be increased, the dielectric layer is often broken due to worn off by rubbing with fingers, etc. In a case where a huge amount of charge is charged, even though the first example of related art in which the sensing electrode is separated from the layer in which the conversion circuit and the wiring are formed, some instantaneous current often flows as long as they are electrically connected. As a result, it is not enough for protective measures.
Also, the second example of related art, in which the conductor or caver is provided so that the target object is contacted before contacting the sensor, needs to form additional components. The components for discharge, which are used for fingerprint sensor to which large pressing force is applied, are affected by the force to easily be broken. Once a part of the components for discharge is broken, the instantaneous current flows directly into the wiring and peripheral circuits, resulting in the breakdown of the peripheral circuits. Accordingly, it is not always preferable that charges generated in the target object are discharged by depending on an electrode arrangement or components for discharge.